Electricity produced via coal combustion results in the emission of gaseous and particulate mercury, a substance that is know to bio-accumulate and cause adverse reactions when consumed. It is very difficult to capture and remove the mercury contained in the flue gas due to the high flue gas velocities, and the low concentrations of mercury contained in the flue gas, as well as the high gas temperatures, the presence of many other complicating compounds in the flue gas, and the fact that multiple mercury species have to be sequestered.
Each of these gas or liquid streams has different characteristics that make some mercury removal methods effective and appropriate, but others, ineffective and inappropriate. Consequently, over the years, a multitude of approaches have had to be developed for effectively removing mercury species from various streams. These overall approaches include, among others: liquid scrubbing technologies, homogenous gas-phase technologies, metal amalgamation techniques, and processes utilizing various sorbent materials in different application schemes, with adsorbents optionally impregnated with various reaction aids.
In the past, activated carbons have demonstrated utility for sequestering mercury vapors in some applications. When combined with halogen compounds, especially iodine, the mercury sequestration performance of activated carbons can be improved.
U.S. Pat. No. 6,953,494 (Nelson) describes a method for removing mercury from a combustion gas in an exhaust gas system. The method has the steps of: providing a mercury sorbent, injecting the mercury sorbent into a stream of the mercury-containing combustion gas for a sufficient time to allow at least an effective amount of the mercury and in the combustion gas to adsorb onto the mercury sorbent, and collecting and removing the mercury sorbent from the combustion gas stream. The mercury sorbent is prepared by treating a carbonaceous substrate with an effective amount of a bromine-containing gas, especially one containing elemental bromine or hydrogen bromide, for a time sufficient to increase the ability of the carbonaceous substrate to adsorb mercury and mercury-containing compounds. The carbonaceous substrate is preferably acticated carbon. A critical element in the process is that a bromine-containing gas is used to treat the carbonaceous substrate.
U.S. Pat. No. 6,027,551 (Hwang et al.) describes a process where mercury emission from a flue gas such as that generated by a coal fired power plant is controlled by injecting into the flue gas unburned carbon purified from ash such as fly ash or wood ash. The unburned carbon adsorbs the mercury and is later removed from the flue gas by a particle separator. The unburned carbon collected from ash is significantly lower in cost compared to activated carbon conventionally used in such a process. The unburned carbon is concentrated in the sorbent by one or more separation processes used to remove non-carbon particles from the fly ash. These processes include gravity separation, electrostatic separation, froth flotation, magnetic separation and size classification. Mercury adsorption is further increased by oxidation of the carbon surface.